Abstract Dysfunction of the serotonergic system is critically involved in a diverse range of diseases, which accumulatively affect at least 20% of the population. In our preliminary studies, we identified transcription factors that converted human fibroblasts to induced serotonergic (i5HT) neurons. The efficiency was significantly increased by p53 knockdown and appropriate cell culture conditions. At day 12 of reprogramming, 50% of the cells were Tuj1+ neurons and 25% were 5HT+ neurons. This epigenetic reprogramming was dependent on Tet proteins, a family of three DNA hydroxylases that critically regulate the epigenome. Knocking down each of the Tet genes abolished the epigenetic conversion. We hypothesize that p53, Tet proteins, and appropriate extracellular environment are critical for the direct conversion human fibroblasts to i5HT neurons. To test this hypothesis, we will identify the optimal transcription factor combinations, investigate how p53 knockdown induces Tet genes, and study how Tet proteins and reprogramming factors impact on the transcriptome to facilitate the transdifferentiation of human fibroblasts to i5HT neurons. We will also examine the impact of cell culture environment on the conversion and assess the survival and function of i5HT neurons transplanted in rat brains. The proposal will develop a robust method for the generation of patient-specific and subtype-specific i5HT neurons, which would enable basic research and drug discovery on serotonin-related disorders.